Diesel engines rely on fuel as a lubricant for their internal moving components. The lubricity of the fuel affects the wear between two metal parts that are in contact. Wear due to the friction between these parts will cause failure of the components if there is insufficient lubricity. The use of a high lubricity fuel may reduce the wear and increase component life.
Lubricity is defined as the property of a lubricant that causes a difference in friction under conditions of boundary lubrication when all the known factors except the lubricant itself are the same. The lower the friction the higher the lubricity. [Kajdas, C., S. S. K. Harvey, and E. Wilusz, Encyclopedia of Tribology, Elsevier, N.Y., 1990.]. One method of testing lubricity is referred to as the Ball on Three Disks method (BOTD). The BOTD system is used to evaluate the lubricity of fuel used in diesel engines.
The main components of a typical BOTD system are:
1) a lubricity test machine (for example, Falex) with timer and hollow (internal taper) rotating shaft which houses the ceramic test ball
2) A two-piece fuel reservoir (upper and lower halves) which when assembled houses a teflon anti-vibration pad, metal disk holder, three steel test disks and the test fuel
3) a lever arm with fulcrum, weight and small metal ball at one end which is used to apply a force to the bottom of the 2 piece fuel reservoir.
Upon completing an intensive cleaning procedure the BOTD components are assembled for testing The anti-vibration pad is inserted into the inside bottom of the lower portion of the fuel reservoir. The disk holder is then placed on the anti-vibration pad and the 3 test disks are place in the disk holder. The ceramic test ball is placed in the lower end of the hollow (internal taper) shaft of the test machine.
The upper portion of the fuel reservoir is then threaded into the lower portion of the fuel reservoir. The upper portion of the fuel reservoir contacts the disk holder centering it and preventing it from rotating during the test. The test fuel is then added through a large opening in the center of the top of the upper portion of the fuel reservoir using the glass syringe and 5 micron Teflon™ filter.
the base of the fuel reservoir, which has a small depression or socket in the center, is then placed on top of the small metal ball on one end of the lever arm. The lever arm is then placed in the fulcrum with the weight nearest the fulcrum. The lever arm and fuel reservoir assembly is then rotated such that the test machine shaft and test ball assembly passes through the large opening in the top of the fuel reservoir until the test ball contacts the test disks.
Once the assemblies are in place as described above, the timer on the test machine is set for 45 minutes, and the start button is pressed causing the test machine ball and shaft to rotate with the ball contacting the disks during rotation. The weight on the lever arm is then gradually pulled towards the outer end of the lever arm, thereby increasing the force applied to the fuel reservoir at the opposite end of the lever arm and correspondingly the contact force between the disks and ball.
Upon completion of the test the assemblies are disassembled and the disks removed for evaluation. Due to the relative motion and contact force between the ball and disks during the test a wear scar is generated on the metal disks. The fuel in the fuel reservoir provides some measure of lubrication between the ball and disks. As such the size of the wear scar on the disks is related to the lubricity characteristics of the fuel. The disks are placed in a holder, and a traveling microscope with dial indicator is used to determine the diameter of the wear scar on the disks. A large wear scar indicates a fuel with poor lubricity and a small wear scar indicates a fuel with good lubricity.
It has been found that this method has the tendency to produce inconsistent results. The reason why has not been apparent.